This invention relates to the manufacture of additive concentrates, especially coloured additive concentrates, for use in the manufacture of thermoplastic articles.
Known colour concentrates consist of a polymer resin composition containing high concentrations of pigments or dyes which is usually in the form of pellets or in diced form. (See, for example, Modern Plastics, Mid-October 1991, pages 155, 156 and 158.) The concentrates can comprise relatively large particles, for example fibre in flock form or flake material such as mica. A fabricator can make products of desired colour and appearance in a simple and reproducible manner by blending a polymer resin, also known as a let-down resin, with a small proportion, for example up to about 4% by weight, of an additive concentrate, also known as a masterbatch. Processing is easier and less messy when masterbatch pellets are used rather than colourants or other additives in liquid or dry form. The use of masterbatches also facilitates handling during storage, transport and weighing. Such known concentrates are typically made by melt-compounding the colourant or other additive into the polymer resin carrier using an intensive mix such as a screw extruder, Banbury-type mixer or compounding mill. Colour concentrates typically comprise 50 to 80% by weight pigment or dye. Concentrates which comprise large particles generally contain lower concentrations because of compounding difficulties.
Thomas B Reeve, in an article entitled "Studies in Pigment Dispersion for Plastics" published in Plastics--Meeting Challenges of the Future, ANTEC '82, 40th Annual Technical Conference & Exhibition, San Francisco, Calif., May 10-13, 1982, pages 389-392 (RAPRA Abstract 229169), describes a process in which pigments, titanium dioxide and resin were dry-blended to produce a tint concentrate which was subsequently extrusion compounded to produce a colour concentrate. If the extrusion compounding step was omitted, colour strength development was considerably reduced and a much greater degree of specking was also observed.
U.S. Pat. No. 5,187,202 discloses a pelletised colour concentrate which comprises up to about 80% by weight of a thermoplastic polymer, at least about 5% and up to about 30% by weight of a cellulosic short fibre or flock, and up to about 10% by weight of at least one dispersant aid. The concentrate may be made by intimate mixing of the components in known types of high-shear mixer or extrusion equipment.
It is an object of the present invention to provide a method of producing additive concentrates which can comprise high levels of large particles, particularly in the form of fibres or flakes.